Compression ratio from compression test?

[TurboDiverArt]

Hi All,

I know it's not an exact science but I was wondering if there was a formula that gets you in the ballpark? I figure with a known cylinder volume and a compression reading one should be able to get in the ballpark with a compression test reading.

I have heard that an 18:1 engine has a compression test reading of about 350 psi. No matter how I slice it I can't seem to come up with a formula that comes close to 18:1. Again, I know it's not an exact science as you really need to take into consideration head gasket thickness, head volume, etc but a simple compression test should be able to get you close I think? Any one know of a ball bark formula?

Art.

[JohnC]

Too many variables. You might be able to come up with a formula for a given engine configuration, but things like cam lift and overlap and cranking speed will skew the results more than the compression ratio will.

[DmaxMaverick]

The C/R doesn't matter. The displacement is the same. A compression test measures the pressure the cylinder will develop. A lower C/R only changes the combustion chamber volume, not the volume of the displacement. Unless you change the bore and/or stroke, the cylinder pressure should not differ on a compression testing gage. There will be a compressible volume variation, but it is negligible for all intents and purposes.

[gmctd]

You can see the effect of that as you run a compression check on any cylinder - hit the starter, note the pressure at the first puff, watch it increase to some point B4 the 10th puff - with a good engine, of course.